An image sensor for an electronic camera ordinarily contains a two-dimensional array of light sensitive photosites. In an interline transfer sensor, the image charge is transferred from the photosites to light-protected vertical registers, through which the image charge is moved to a horizontal readout register. On the other hand, in a full frame image sensor the image charge is incremented line-by-line through the photosites themselves to a horizontal readout register, from which the image signal is obtained. Since the full frame image sensor does not contain a separate light-protected storage area, a shutter is normally used to block the light and prevent vertical smearing when reading out the image.
An example of an electronic still camera is the model DCS 200c camera, marketed by Eastman Kodak Company, Rochester, N.Y. This camera uses a high resolution full frame image sensor in an electronic camera back which attaches to a conventional 35 mm film camera body. The camera body includes a normal focal plane shutter for blocking image light. In this type of electronic camera, a separate focus sensor is used in the camera body to automatically focus the camera lens. This extra sensor increases the camera cost. It may also result in a less than optimally focused image on the sensor, if the tolerances of the camera lens focusing mechanism, and the tolerances of the focus detector, are not tightly controlled. Therefore, by using the high resolution image sensor itself to focus the camera lens, the camera cost may be reduced, and the camera focusing accuracy may be increased.
Focusing a camera lens by using the sensor output signal is an iterative process which requires capturing a sequence of images while varying the focus, until a focus-related parameter of the image, such as the "average contrast", is maximized. Prior art camcorders typically use NTSC format interline image sensors, which do not require a mechanical shutter, to perform this type of automatic lens focusing. In these camcorders, the same image that is recorded is also analyzed to provide the focus information. Focusing is done by spatially bandpass filtering a subsection of the video image read out from the sensor. The lens focus position is adjusted to obtain the highest average magnitude output signal (highest average contrast) from the bandpass filter. Note that the camcorder's image sensor is ordinarily read out at the video field rate (1/60 second) so that the same sensor operating mode is used to provide both the focus information and the final images. In such camcorders, many images may need to be read out before the lens is properly focused. Since the image readout time is relatively rapid (approximately 1/60 second), acceptable focus can typically be achieved in less than a second.
Focus time can be further improved by application of the technique shown in U.S. Pat. No. 5,051,833 (Tsuji). This patent describes an electronic still camera in which focus is based on a rectangular subset of the pixels on an interline charge coupled device (CCD) sensor. During the focusing operation, the image lines within the rectangle are read out at the regular NTSC video rates, while the lines outside the rectangle are read out more rapidly to a charge drain on the CCD sensor. Tsuji thus describes a focusing mode which takes less time for reading out an image for the purpose of focusing than is taken for reading out an image for normal image capture. This focusing mode is employed with an interline transfer sensor, which has light-protected storage areas (vertical registers) and consequently does not require a mechanical shutter. Because the focusing image frames are not usable as captured images, this technique is not useful with camcorders. Moreover, focusing time can remain a problem if rapid utilization of the camera is desired or if longer readout times are required, such as for definitions higher than NTSC resolution.
Progressive scan image sensors having a noninterlaced architecture, such as the Kodak model KAI-0310CM imager, have been developed for high quality color electronic cameras. This sensor has approximately 480 active lines, and approximately 640 active pixels per line. A progressive scan sensor provides a higher quality still image than an interlaced sensor, since all lines are captured during the same interval of time. Unlike an interlace sensor, the progressive scan sensor allows the entire image to be read out in a single scan, albeit through light protected vertical registers as in the interline sensor. Mechanical shuttering, therefore, is unnecessary for exposure control (and to prevent vertical smear) as the function can be performed electronically.
For a high quality still mode, the progressive scan sensor can be read out at slower than video rates without sacrificing performance, since real-time operation is not required. If, however, the image is read out in the same period of time as from an interlaced image sensor, readout timing from a progressive scan image sensor normally requires a clock rate of approximately twice that used with the interlaced image sensor. Such timing would be required, for instance, for an electronic viewfinder or for the rapid acquisition of image data, such as for autofocus determination. For example, an NTSC format interlaced image sensor with 480 lines and 640 pixels per line requires a clock rate of approximately 12 MHz to read out the 640 lines in the 52.4 mSec NTSC standard active line time, which provides a field rate of 1/60 second. Since a progressive scan sensor, like the model KAI-310 imager, must read out twice as many lines per field, it must use a clock rate of about 24 mHz to read out all 480 lines in 1/60 second. This higher clock rate requires more expensive clock drivers, analog processing, and A/D conversion than interlaced sensors require. Such high speed, and thus high cost, components are required for autofocus processing within the focusing area even though, as in Tsuji, the lines above and below the focusing area are not used.
This leads to the anomalous situation that a non-imaging part of the system, the autofocus processing, requires the higher clock rate, and thus the higher cost parts--clock drivers, analog processing, A/D converters, etc.--than the high resolution imaging part itself. What is needed is a technique for reading out the image sensor data within the focusing area in a manner that decreases the required clock rate while enabling rapid focus of the lens of an electronic camera.